I use Screamers from time to time. But they're not flying meltaguns. They're flying melta bombs. Meltaguns have AP 1, melta bombs do not. Meltaguns can be shot from a distance, meltabombs require base-to-base assault. Now, if assaults could be declared one at a time, rather than all at once, Screamers would be far more useful in opening up vehicles.
Math that assumes combat speed is not as practical as math based on cruising speed. I have to assume my Daemons need 6s to hit, because my opponents routinely exploit this. If I have three Screamers attacking a Rhino/Razorback, that's a 1/2 chance I'll hit, all hits will damage and basically 1/3 of those results will wreck or explode the vehicle. That's not exactly reliable, and anything short of that results in them getting lit up next round, dying a senseless death.
Screamers vs AV 14, that has mixed results for me. Basically, it's the same problem of hitting the vehicle on 6s, with less of a chance of crippling the vehicle. It's like hammering in a screw; it may get the job done, but it will never be pretty.
Screamers do have a couple other not-immedialely-obvious uses, though.
With a 4+ invulnerable save, sometimes they can occupy some rather tricky solo assault models, like Walkers or Mephiston. I'll aim for Walkers, because they are a really troublesome match-up for the Elite slots, which are generally the greatest strength in any Daemons list.
As they're jetbikes, I sometimes like Screamers to hold back and turbo-boost to contest objectives late-game.
Whatever you use them for, they're one-use units. They go in, take their chances and do not live to see the next turn. Having disposable units is not a bad thing; sometimes just counter-intuitive.
Screamers work much better when Skarbrand is on the field, but that's a whole other discussion. I like Skarbrand, but would competitively run many other units above him.
Also, don't put Screamers in Rapid Fire range! They have 12" movement and a 6" assault. You have to anticipate your opponent's movement and hopefully not scatter too far on your Deep Strike...
